You [highlight]SHOULD NOT [/highlight]rout / cut / notch the [highlight]top or bottom cord [/highlight]of any engineered "I" joist . That's where the strenght is , read the instruction before modifing them , even holes in the webbing need to be in the "right" place and there are size of hole requirements as well.
AS Glenn mentioned there are systems designed to mount the tubing on the under sides.
You be advised to dry that T&G very well before installing the flooring , it will be very dry once the system is turned on , as low as 2 or 3 % MC, so when it's installed it should be at least 6% or lower or it will shrink up to open gaps between the boards.
If it a seasonal cabin , it would be a poor choice of heating , and the flooring will be aways moving if it's unheated part of the year.